During semiconductor manufacturing and the like, it is necessary to measure device structures to ensure the manufacturing accuracy. The manufacturing accuracy, such as dimensional control of structures, is necessary to achieve the desired performance of the finished device. Optical metrology is particularly well suited for measuring device structures because optical metrology is nondestructive, accurate, repeatable, fast, and inexpensive.
One type of optical metrology tool that may be used, particularly for the measurement of sub-resolution diffracting structures, is a scatterometer. Scatterometry measures structures using light that is scattered from the structure, sometimes referred to as a scatter signature. With scatterometry, modeling techniques are used in which the sample is modeled and the resulting scatter signature is mathematically calculated from the model using, e.g., a rigorous numerical method. When the calculated scatter signature has a good fit with the scatter signature measured from a sample, the model is considered to be an accurate description of the sample. If the calculated scatter signature does not fit well with the measured scatter signature, one or more variable parameters in the model may be adjusted and the resulting scattered light calculated for the adjusted model. The model may be repeatedly adjusted until the fit between the calculated scatter signature and the measured scatter signature are within tolerance. In some systems, multiple varying models may be pre-generated and stored, along with their associated calculated scatter signature, in a library that is consulted during the measurement of a sample.
Modeling techniques, including calculating the resulting scatter signatures, are particularly useful when the sample to be measured is a simple structure, such as a pattern that is periodic in one direction, e.g., lines and spaces. Unfortunately, when the sample is complicated, such as being periodic in two different directions, modeling the sample can be difficult, time consuming and require a large amount of memory.